Blends of a polyphenylene ether resin and a styrene type resin are known in the art and described in Cizek, U.S. Pat. No. 3,383,435, incorporated herein by reference. Th polyphenylene ether resin contains repeating structural units conforming to the general formula: ##STR1## wherein the ether oxygen atom of one unit is connected to the benzene nucleus of the next adjoining unit; n is a positive integer equal to at least 50 and each of Q and Q.sup.1 is a monovalent substituent selected from the group consisting of hydrogen, halogen, hydrocarbon radicals, halohydrocarbon radicals, hydrocarbonoxy radicals and halohydrocarbonoxy radicals provided that none of said radicals contains an alpha-carbon atom which is tertiary or bears a halogen substituent. Examples of such polyphenylene ethers and methods for producing same may be found in Hay, U.S. Pat. No. 3,306,874 and 3,306,875, and Stamatoff, U.S. Pat. No. 3,257,357 and 3,257,358, each of which is incorporated herein by reference.
The styrene type resin, as is described in the above noted Cizek patent, has at least 25 percent by weight polymer units derived from a compound corresponding to the formula: ##STR2## where R' is hydrogen, lower alkyl or halogen, Z is a member of the group consisting of vinyl, halogen and lower alkyl; and p is a whole number equal to from 0 to 5. Typical styrene type resins include, by way of example, homopolymers such as polystyrene and polychlorostyrene, and the modified polystyrenes such as rubber modified polystyrene (high impact polystyrenes), and styrene containing copolymers such as the styrene-acrylonitrile copolymers (SAN), styrene-butadiene copolymers, styrene-acrylonitrile-alpha-alkyl styrene copolymers, styrene-acrylonitrile-butadiene copolymers (ABS), poly-alpha-methyl styrene, copolymers of ethylvinyl benzene and divinyl benzene, and the like.
The inadequate flame retardant properties of thermoplastic compositions produced by blending styrene resins with polyphenylene ether resins have been recognized heretofore, and attention has been directed to the use of flame retardant additives therein. For example, Haaf, U.S. Pat. No. 3,639,506, incorporated herein, describes the ability of various flame retardant additives to render such polymer blends "self-extinguishing" and "non-dripping" according to ASTM test method D635 and Underwriter's Laboratories Bulletin No. 94. However, said Haaf patent also discloses that other properties of such polymer compositions, such as their "heat distortion temperatures", are adversely affected by the presence of effective amounts of the chosen flame retardant additives, particularly those of the aromatic organic phosphate type, e.g., triphenyl phosphate.
A primary object of the present invention is to provide thermoplastic compositions based upon compositions of styrene type resins and polyphenylene ether resins in which increased flame retardant performance is achieved while maintaining an overall balance of physical properties.
Another object of this invention is to provide highly flame retardant resin compositions of the subject class which exhibit unexpectedly high heat distortion temperatures under load (ASTM D648). A further object is to provide practical molding compositions of the subject class which can be readily fabricated into finished articles with well balanced physical properties.